Cryptology ePrint Archive: Report 2010/203

Heraclitus: A LFSR-based Stream Cipher with Key Dependent Structure

Bernard Colbert and Anthony H. Dekker and Lynn Margaret Batten

Abstract: We describe Heraclitus as an example of a stream cipher
that uses a 128 bit index string to specify the structure of each instance in real time:
each instance of Heraclitus will be a stream cipher based on mutually
clocked shift registers.

Ciphers with key-dependent structures have been investigated and are
generally based on Feistel networks. Heraclitus, however, is based
on mutually clocked shift registers. Ciphers of this type have been extensively analysed, and published
attacks on them will be infeasible against any instance of Heraclitus.

The speed and security of Heraclitus makes it suitable as a session cipher,
that is, an instance is generated at key exchange and used for one session.